1. Field of the Invention
The present invention relates to a press sheet engraving apparatus for forming a press sheet of a gravure printing press, for example.
2. Description of the Related Art
A press sheet engraving apparatus has hitherto been proposed to engrave a press sheet by forming concavities corresponding to image information on a resin sheet with radiation of a laser beam on the resin sheet while a cylinder on which the above resin sheet is wrapped is rotated at a constant speed.
The press sheet corresponding to the image information is engraved by forming concavities having different volumes at the unit of a pixel while a radiation time of a laser beam to be irradiated is varied in response to the light and shade (density) of the image information.
In the gravure printing press, the light and shade of the image information, i.e., image density, is determined by "opening area x depth=volume" of concavity formed at the unit of a pixel. In the gravure printing press, for example, it is customary that image information is printed by filling a printing ink in response to the volume of the concavity. Thus, density depends upon a quantity of filled printing ink, i.e., the volume of concavity.
Therefore, if power, i.e., intensity, of laser beam is constant and a rotational speed of the cylinder, in other words, a relative scanning speed of a laser beam on the sheet is constant, then the depth of concavity becomes constant and a density of image information becomes proportional to the opening area of the concavity.
For this reason, when the press sheet is formed by radiating a laser beam on the sheet, from the above-mentioned point of view, "relative scanning speed x laser power" is set such that an opening area and depth of a concavity of pixel data having a maximum density of pixel data representing image information (hereinafter simply referred to as image data when necessary) may become maximized. This setting value is generally determined by a specification (performance) of the press sheet engraving apparatus.
By varying the laser radiation time per pixel in response to a density of each of the pixel data constructing image data on the basis of the set value, concavities having different opening areas corresponding to predetermined densities are formed with a constant depth, thereby engraving the press sheet. The assignee of the present application has also filed U.S. patent application Ser. No. 07/899,307 now abandoned with respect to the gravure printing press.
In the prior art, the whole of the sheet is engraved by one scanning. More specifically, the whole of the sheet is scanned by sequentially carrying out a process in which the main scanning direction of the next line is engraved by shifting a laser light source to a sub-scanning direction by one line (one pixel amount) at the completion of one line (one revolution) while one line in the main scanning direction is engraved when the cylinder is rotated once. In this case, it is requested that a total engraving time per sheet, i.e., the time corresponding to one scanning, should be reduced.
In the conventional apparatus, although the laser radiation time per pixel is different when a concavity (pixel) having a small opening area is engraved or when a concavity (pixel) having a large opening area is engraved, the engraving time (scanning time) per pixel becomes the same time and a total engraving time becomes the same time. There is then the problem that a lot of time is wasted.